Presentation API control protocol

[mfoltzgoogle]

This work item covers the control protocol for the Presentation API, i.e. the syntax and meaning of the messages sent over the transport [1] to start and control the presentation.

It might make sense to pick an initial transport before undertaking this work item, since the format of the control messages may be tied to the transport. For example, we may be able to map control messages onto transport level mechanisms like QUIC resource streams.

[1] #8 Evaluate protocol transport mechanisms

[mfoltzgoogle]

I'm closing this issue since we have a working definition drafted by the three pull requests for the Presentation API. There are several TODOs in the document tracked under other GitHub issues. I'll file one issue more to link specific messages to steps in the Presentation API spec so it's clearer how the semantics of the API map onto the control protocol.